Physics

Pascal (unit) The pascal (Pa) is the SI derived unit of pressure and stress, defined as one newton per square metre. Used in hPa and kPa for weather, engineering and scientific measurements.

Pascal's law Pascal's law: in a confined, nearly incompressible fluid, an applied pressure change is transmitted undiminished. It underpins hydraulic machines, jacks, brakes and many engineerin…

Pauli exclusion principle The quantum rule that no two identical fermions in the same quantum system can occupy the same quantum state; key to atomic structure, chemistry and stellar degeneracy.

Pellet: small compressed particle — definition, types, uses, and production Pellet A pellet is a compacted small particle or grain formed by pressing or agglomerating material. Used in fuel, feed, industry and pharmaceuticals, pellets are valued for uniformity, h…

Pendulum: motion, parts, formula, history and applications Pendulum A pendulum is a weight suspended from a pivot that swings under gravity. This article explains parts, motion, period formula, energy, historical milestones and common uses includin…

Penetration power (projectiles and barriers) Penetration power Penetration power describes how deeply a projectile or weapon can pierce a target. It depends on velocity, mass, shape, construction and the target material, and is central to ball…

Period (physics) The period is the time required for one complete cycle of a repeating motion or wave. It is measured in seconds and is the reciprocal of frequency.

Magnetic permeability (electromagnetism) Permeability (electromagnetism) Permeability (μ) quantifies how a material supports the formation of magnetic fields. It links magnetic field strength and flux density and varies by material, frequency and magnet…

Permeance: how easily matter or energy passes through a medium Permeance Permeance measures how readily a substance, flux or energy flows through a component or material. Used in magnetics, vapor transport and membrane science; distinct from intrinsic p…

Perpetual motion Perpetual motion refers to hypothetical devices or processes that would run indefinitely without external energy input, violating conservation of energy and thermodynamic laws; mod…

Phase (cyclic states in nature and science) Phase A phase is a particular state within a repeating cycle. This article explains phases in waves, astronomy, matter, and engineering, and how phase differences affect behavior and mea…

Phase difference Measure of how far two periodic signals or oscillations are out of step, expressed as an angle, time delay, or distance; central to interference, synchronization and signal analysi…

Phase velocity Phase velocity is the speed at which a particular phase of a single-frequency component of a wave propagates; distinct from group velocity and affected by dispersion and the medium…

Phenomenon: definition, origins, types, and uses Phenomenon An overview of phenomena: observable events or occurrences, their etymology, types, examples, and how the term is used across science, philosophy, and everyday language.

Philosophiæ Naturalis Principia Mathematica Isaac Newton's 1687 Principia sets out the laws of motion and universal gravitation in three books, founding classical mechanics and reshaping natural philosophy with mathematical…

Phonon — the quantized vibration of a crystal lattice Phonon A phonon is the quantum of vibrational energy in a crystal lattice: a quasiparticle describing collective atomic motion, important for thermal, electrical and optical properties of…

Phosphorescence: slow photoluminescence and glow-in-the-dark materials Phosphorescence Phosphorescence is a form of photoluminescence in which absorbed energy is re-emitted as light over an extended period. It differs from fluorescence and has practical uses in safet…

Photocathode: light-sensitive electron-emitting electrode Photocathode A photocathode is a photoemissive electrode that releases electrons when struck by light. It is a core component of photomultipliers, night-vision devices and various photon-detect…

Photoelectric effect: emission of electrons induced by light Photoelectric effect The photoelectric effect is the emission of electrons from matter when it absorbs electromagnetic radiation. It revealed light's quantum nature and underpins devices from photodiod…

Photographic filter: types, uses, history, and practical guidance Photographic filter An overview of photographic filters: what they are, common types (UV, polarizing, ND, color, diffusion, close‑up), materials, mounting, uses in film and digital photography, and bu…

Photon (elementary quantum of light) Photon A photon is the quantum of the electromagnetic field: a massless, spin-1 boson that carries light, energy E=hf, and momentum, and underlies optics, lasers and quantum communication…

Physical (term): meanings, uses, and distinctions Physical Comprehensive overview of 'physical' as an adjective and noun: relations to the body and material world, scientific senses, medical and legal uses, philosophical positions, and con…

Physical coefficient A physical coefficient is a numeric factor that quantifies a relationship between physical quantities; it can be empirical or derived, dimensional or dimensionless, and is central…

Physical compression Reduction of a material's volume or increase in pressure when subjected to compressive force; principles, behavior in solids and gases, measurement methods, and engineering applica…

Physical constant (fundamental physical constants and their role in science) Physical constant A physical constant is a measured quantity believed to be universal and invariable. This article explains types, examples, how they are measured, their role in units and theories,…

Physical fitness: components, measurement, history and practical importance Physical fitness Concise guide to physical fitness: definition, key components (endurance, strength, flexibility, composition), common assessments, historical context, occupational standards, and d…

Physical law: principles describing regularities in nature Physical law An overview of physical laws: concise, empirical descriptions of regularities in nature, how they are formulated, tested, interpreted, and distinguished from legal or philosophical…

Physical object (in physics and everyday use) Physical object A physical object is a collection of matter treated as a single entity. This article explains definitions, properties like mass and boundary, examples from atoms to galaxies, and r…

Physical property A physical property describes measurable characteristics of matter that do not alter its chemical identity. Includes intensive vs extensive types, common examples, measurement, and…

Physical quantity A physical quantity is any measurable property of a physical system. This article explains definitions, base and derived quantities, units and dimensions, measurement, history, and…

Physical system (physics) Physical system A physical system is a chosen portion of the physical world treated as a unit for analysis under the laws of physics, separated conceptually from its environment to study behaviour…

Physicalism: the philosophical view that reality is fundamentally physical Physicalism Physicalism is the metaphysical position that everything that exists is physical or depends on the physical; it appears in debates about mind, science, and ontology and contrasts w…

Physicist: Role, Methods, History, and Notable Figures Physicist Overview of physicists: what they investigate, how they work, historical development, common career paths, and several well-known figures in the field.

Physics: study of matter, energy, space and time Physics Physics examines matter, energy, forces, space and time. It develops laws and models from classical mechanics to quantum theory and underpins many technologies.

Physique — human body build, types, and significance Physique Overview of human physique: definitions, common classification systems, historical background, practical importance in health, sports, fashion and related distinctions.

Picometre (pm) — SI unit of length equal to 10⁻¹² metre Picometre A picometre (pm) is an SI unit of length equal to 10⁻¹² m. It measures atomic- and subatomic-scale distances and is used in physics, chemistry, crystallography and nanotechnology.

Picosecond (ps): one trillionth of a second Picosecond A picosecond (ps) is 10⁻¹² seconds. This article explains its scale, notation, common conversions, physical examples, measurement methods and where picosecond processes matter.

Piezoelectricity: principles, materials, history, and applications Piezoelectricity Piezoelectricity is the ability of certain materials to generate electric charge when mechanically stressed, and conversely to deform under an applied electric field. Overview, mat…

Pion (π meson): properties, types, and role in nuclear physics Pion A pion is the lightest meson, made of an up or down quark and an antiquark. Pions form an isospin triplet (π+, π0, π−), mediate nuclear forces, and play key roles in particle decay…

Planck constant Fundamental physical constant linking a photon's energy to frequency; central to quantum mechanics and the modern SI system, often used in the form ħ = h/2π.

Planck length — the fundamental quantum of length Planck length The Planck length is a natural length scale derived from fundamental constants (≈1.616×10⁻³⁵ m). It indicates where quantum gravity is expected to dominate and classical spacetime…

Planck mass The Planck mass is a fundamental mass scale set by Planck’s constant, the speed of light and Newton’s constant. It marks the energy scale where gravitational and quantum effects ar…

Planck time Planck time is the natural unit of time derived from fundamental constants. It marks the scale at which classical ideas of space and time break down and quantum gravity effects are…

Planck units: the natural system of measurement defined by fundamental constants Planck units Planck units are a system of natural units derived from universal physical constants. They simplify equations in theoretical physics and provide characteristic scales for length, t…

Planck's law — spectral distribution of blackbody radiation Planck's law Planck's law describes how a perfect thermal emitter (black body) radiates electromagnetic energy versus frequency or wavelength, introducing energy quantization and underpinning q…

Plane wave An idealized wave whose surfaces of constant phase are infinite parallel planes; a mathematical model widely used in optics, acoustics, electromagnetism, and quantum theory.

Plasma (ionized matter and related concepts) Plasma Plasma is an ionized state of matter whose behavior is dominated by collective electromagnetic effects; it occurs in stars, lightning, neon lights and many technologies, and is cen…

Plasma (physics): the ionized state of matter and its properties Plasma (physics) Plasma is an ionized state of matter whose charged particles respond to electromagnetic fields; it dominates the visible universe and has many natural and technological roles.